Blends of linear polyesters with other incompatible materials of organic or inorganic nature to form microvoided structures are well-known in the art. U.S. Pat. No. 3,154,461 discloses, for example, the linear polyester, poly(ethylene terephthalate), blended with, for example, calcium carbonate. U.S. Pat. No. 3,944,699 discloses blends of linear polyester, preferably poly(ethylene terephthalate) with 3 to 27% of organic material such as ethylene or propylene polymer. U.S. Pat. No. 3,640,944 also discloses the use of poly(ethylene terephthalate) but blended with 8% organic material such as polysulfone or poly(4-methyl,1-pentene). U.S. Pat. No. 4,377,616 discloses a blend of polypropylene to serve as the matrix with a small percentage of another and incompatible organic material, nylon, to initiate microvoiding in the polypropylene matrix. U.K. Patent Specification No. 1,563,591 discloses linear polyester polymers, and particularly poly(ethylene terephthalate), for making an opaque thermoplastic film support in which have been blended finely divided particles of barium sulfate together with a void-promoting polyolefin, such as polyethylene, polypropylene and poly-4-methyl-1-pentene.
The above-mentioned patents show that it is known to use incompatible blends to form films having paper-like characteristics after such blends have been extruded into films and the films have been quenched, biaxially oriented and heat set. The minor component of the blend, due to its incompatibility with the major component of the blend, upon melt extrusion into film forms generally spherical particles each of which initiates a microvoid in the resulting matrix formed by the major component. The melting points of the void initiating particles, in the use of organic materials, should be above the glass transition temperature of the major component of the blend and particularly at the temperature of biaxial orientation.
As indicated in U.S. Pat. No. 4,377,616, spherical particles initiate voids of unusual regularity and orientation in a stratified relationship throughout the matrix material after biaxial orientation of the extruded film. Each void tends to be of like shape, not necessarily of like size since the size depends upon the size of the particle.
Ideally, each void assumes a shape defined by two opposed and edge contacting concave disks. In other words, the voids tend to have a lens-like or biconvex shape. The voids are oriented so that the two major dimensions are aligned in correspondence with the direction of orientation of the film structure. One major dimension is aligned with machine direction orientation, a second major dimension is aligned with the transverse direction orientation, and a minor dimension approximately corresponds to the cross-section dimension of the void-initiating particle.
The voids generally tend to be closed cells, and thus there is virtually no path open from one side of a biaxially oriented film to the other side through which liquid or gas can traverse.
Upon biaxial orientation of the resulting extruded film, the film becomes white and opaque, the opacity resulting from light being scattered from the walls of the microvoids. The transmission of light through the film becomes lessened with increased number and with increased size of the microvoids relative to the size of a particle within each microvoid.
Also, upon biaxial orientation, a matte finish on the surface of the film results, as discussed in U.S. Pat. No. 3,154,461. The particles adjacent the surfaces of the film tend to be incompressible and thus form projections without rupturing the surface. Such matte finishes enable the film to be written upon with pencil or with inks, crayons, and the like.
Although the films discussed so far are generally white and opaque, suitable dyes may be used either in what will become the matrix polymer or in the void initiating particles. U.S. Pat. No. 4,377,616 points out that interesting effects can be achieved by the use of spheres of different colors or by the use of spheres of different color absorption or reflectance. The light scattered in a particular void may additionally either be absorbed or reflected by the void initiating sphere and a separate color contribution is made to the light scattering in each void.
U.S. Pat. No. 4,377,616 discloses that preferred particle size of a void initiating sphere may be about 0.1 to about 10 microns, and that preferred particle size range from about 0.75 to about 2 microns. U.S. Pat. No. 3,154,461 specifies that a range of sizes may be approximately 0.3 micron to approximately 20 microns, and that when calcium carbonate is used, its size may range from 1 to 5 microns.
U.S. Pat. No. 3,944,699, for example, indicates that the linear polyester component of the film may comprise any thermoplastic film forming polyester which may be produced by condensing one or more dicarboxylic acids or a lower alkyl diester thereof, such as terephthalic acid, isophthalic acid, 2,5-, 2,6- or 2,7-naphthalene dicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, bibenzoic acid, and hexahydroterephthalic acid, or bis-p-carboxy phenoxy ethane, with one or more glycols. Such glycols may include ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, and 1,4-cyclohexanedimethanol. Also, a copolyester of any of the above-indicated materials may be used. The preferred polyester is poly(ethylene terephthalate).
U.S. Pat. No. 3,944,699 also indicates that the extrusion, quenching and stretching of the film may be effected by any process which is known in the art for producing oriented film, such as by a flat film process or a bubble or tubular process. The flat film process involves extruding the blend through a slit dye and rapidly quenching the extruded web upon a chilled casting drum so that the polyester component of the film is quenched into the amorphous state. The quenched film is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature of the polyester. The film may be stretched in one direction and then in a second direction or may be simultaneously stretched in both directions. After the film has been stretched it is heat set by heating to a temperature sufficient to crystallize the polyester while restraining the film against retraction in both directions of stretching.
Paper is essentially a non-woven sheet of more or less randomly arrayed fibers. The key properties of these structures are opacity, texture, strength, and stability. Obviously, fiber technology evolved synergistically with paper, and today we have a variety of synthetic fibers and synthetic papers. In both areas, however, the synthetic materials have never quite matched the cellulose-based natural polymers, like cotton for fibers and cellulose pulps for papers. On the other hand, the natural polymers are generally weaker and less stable. A serious problem, for example, is brightness reversion or fading of papers and fibers. The present invention advances the state of these prior arts.
Although there are many ways to produce opaque media, this invention is concerned with creating opacity by stretching or orienting plastic materials to induce microvoids which scatter light, preferably white light. A large body of prior art deals with this technique, wherein a plurality of inorganic solid particles are used as the dispersed phase, around which the micovoids form. Some significant problems associated with this approach are: (1) agglomeration and particle size control, (2) abrasive wear of extrusion equipment, guides, and cutters, (3) high specific gravity of these solids, (4) poor void nucleation around the solid particles due to the low thermal contraction of solids relative to liquids and polymer wetting and adhesion to the solid surfaces, (5) cost of these materials on a volume basis, and (6) handling and processing problems in general. In every case, the invention reduces or eliminates the problem.
The prior art also teaches a variety of methods of creating surface texture. Often the surface is roughened by physical means like abrasion, crimping, etc. Many chemical methods are also used to react with, etch, or otherwise alter the surface. Flame, electrical corona, and electromagnetic radiations are often employed. Coating technology is well advanced for filling and whitening, and often inorganic materials are major components of these coatings. Even if the orientation or stretching step is eliminated, a coating step is required. Not only do most of the problems above remain, but new ones are created in such areas as adhesion, uniformity, and coating stability.
The cited prior art concentrates on synthetic paper compositions and methods of manufacturing directly related to this invention, namely compositions of matter involving polyesters and/or cellulose esters, stretching incompatible/immiscible thermoplastic blends to create voided structures with or without texture, and some of the properties and problems associated with the use of inorganic, nonmelting materials. The blend compositions and processing methods of this invention constitute a significant improvement over the immiscible polymer blend systems found in the prior art.